Although utility companies and municipalities attempt to chart the location of their service lines, their exact location is frequently not known, at least to the extent they may be easily exposed by excavation. To avoid the expense of multiple excavations where there is a need to access such lines or to simply obtain accurate information as to their location, instrumentation has been developed for such purpose.
Several methods have been developed for directly tracing the location of buried metal utility pipelines; methods to directly detect or trace the location of buried plastic pipelines have met with limited success. For this reason, it is usual practice to bury a metal wire or foil over the plastic pipeline that can be detected using conventional electromagnetic metal pipe locating techniques. This procedure is both costly and inefficient as the wire or foil can be cut by later construction activity, deteriorate with time, or may not be located directly over the pipeline.
Other methods including downward looking radar have been developed to directly locate buried plastic pipelines. These methods have also met with limited success.
Acoustic methods have been investigated to develop better techniques for pinpointing leaks in gas pipelines. For example the instrumentation of U.S. Pat. No. 3,055,209 comprises a sound generator 3 disposed to provide single frequency acoustic energy within a pipeline, a sound sensing device 7 on the surface detects the sound. A change in the sound reception indicates a breach 5 in the pipeline. However, the device 7 is moved in contact with the surface parallel to the length of the pipeline so that the location of the pipeline must be known in advance of locating the pipeline. This instrument would not be reliable for locating the pipeline, were its position not known, because only a single frequency within the ranges specified is generated. Such single wavelength is readily distorted by underground obstacles such as rocks or surface obstacles such a curbing, paving and sidewalks. Single frequency methods are further inadequate since the frequency response of different pipe installations or the local response at different locations in the same system vary significantly.
U.S. Pat. Nos. 3,170,152; 3,264,864; and 4,013,905 all describe acoustic devices for determining the position of leaks in buried pipelines or conduits all of which employ the generation of substantially single frequency sound and which accordingly may not be relied upon to find lost or displaced pipelines.